Societal freedom is not contained in any building or exhausted by any particular person. Thus freedom itself cannot be destroyed for society as a whole in virtue of the destruction of a particular building or the murder of a particular person. Concordantly, the more severe danger to freedom comes not from bogeyman "mad bombers" who can threaten only particular buildings and persons, but from perennial (power-)mad rulers and legislators who can threaten the entire society with tyrannical laws.
To the extent that 9/11 represents an attack on freedom, then, it is incomplete and unable by itself to succeed in destroying freedom. Our response completes the attack. If we suffer a segue into police state tyranny, then the attack will be made successful, retroactively - not by the bombers involved in 2001, but by the legislators who operated afterwards. If the attack was an attack on freedom, then we properly oppose the attack by strengthening freedom, multiplying liberties. "Walling in freedom" is tantamount to taking it away. It's as misguided as trying to preserve the life of a delicate flower by encasing it. The act of "shielding" it cuts off it's water and light.
For besides the words used (and superficial things like dress uniforms), what - really - is supposed to be the difference between "freedom in a fortress" and "unfreedom in a prison"? Seemingly, both reduce persons to living in locked boxes, guarded and filmed.
No lone nut bomber can force us to such a state. Therefore, no such bomber is the real threat to freedom.
Aesop: THE CAT, THE COCK, AND THE YOUNG MOUSE